1. Field of the Invention
The present invention relates to a synthetic aperture sonar and a synthetic aperture processing method, and in particular, to a synthetic aperture sonar using an actual aperture array division and transmission and reception multiplexing method, and a synthetic aperture processing method for accelerating a traveling speed.
2. Description of the Related Art
Synthetic aperture radars (SAR) are used in artificial satellites and aircrafts as radar systems with high spatial resolution. What synthetic aperture processing is applied to a sonar on the basis of the same principle as this radar is a synthetic aperture sonar (SAS).
FIG. 8 is an explanatory diagram of the basic principle of a Strip-Map Type synthetic aperture sonar. With referring to this figure, it is assumed that an actual aperture array 101 travels in the direction of the upper part (an azimuth direction) from a lower part of this figure. First, a first transmission and reception of a signal is performed in the direction E of a detection target (a range direction) at a position A, and next, a next transmission and reception of a signal is performed at a point where the sonar travels a half of actual aperture array length. This transmission and reception of a signal is totally performed N times continuously until the sonar reaches a position C through a position B. At this time, a detection target 102 is always included in an emission range of each beam in N-time transmission and reception. Then, by collecting this N-time received data, and performing synthetic aperture processing (convolution processing), it is possible to obtain the same resolution as the resolution obtained when one-time transmission and reception is performed in an aperture length (synthetic aperture length) H longer than that of the actual aperture array 101. This is the basic principle of a synthetic aperture sonar.
Although there are several methods also in synthetic aperture processing, here, an object is a Strip-Map type synthetic aperture sonar that is most common.
The Strip-Map type synthesis aperture sonar can be recognized as means for improving the azimuth resolution of a side-scan sonar (SSS) that has been used until now. The side-scan sonar generates a two-dimensional map of sound reflective intensity of a submarine surface by traveling with continuously performing sound transmission and reception. The resolution in a range direction (range resolution) of this submarine surface map is proportional to pulse length when PCW (Pulse Continuous Wave) is used for transmission and reception signals, or to frequency bandwidth when a wide band signal like LFM (Linear Frequency Modulation) is used. Moreover, angular resolution is determined by the width of a beam that an echo sounder transducer array forms. That is, angular resolution is proportional to an actual aperture length or a center frequency of the sonar.
On the other hand, a synthetic aperture sonar is a method for obtaining angular resolution (or an azimuth resolution) higher than a usual SSS by generating a long virtual array (synthetic aperture array) with using a plurality of continuous transmission and reception signals as described above.
Usually, the synthetic aperture sonar performs the processing of changing the length of a synthetic aperture array in proportion to a range. Consequently, on the submarine map obtained, spatial resolution in the azimuth direction (azimuth resolution) becomes constant regardless of the range. This azimuth resolution is restricted to D/2, that is, a half of actual aperture array length D from grating lobe suppression conditions. This just means that a space-sampling period becomes D/2, and the spatial resolution obtained as the result of performing synthetic aperture processing is also restricted to D/2 or less.
Moreover, letting the maximum range be R, the traveling speed V is restricted as follows:
i Vxe2x89xa6(D/2) (c/2R)xe2x80x83xe2x80x83(1)
Here, item c denotes a underwater acoustic velocity. Although this trade-off condition of V and R can be also applied to a radar, the traveling speed, range, and resolution are restricted severely in comparison with the radar in a sonar due to the slowness of the underwater acoustic velocity c.
Several methods that improve implementation efficiencies of sonars with exceeding this trade-off restrictions are proposed. For example, there are a vernier division method (M. A. Lawlor et al., xe2x80x9cFurther results from the SAMI synthetic aperture sonarxe2x80x9d, IEEE OCEANS""96, 1996, Vol. 2, pp 545-550), a Japanese Patent Laid-Open No. 10-142333, etc. Each of these methods performs a plurality of space sampling by one-time transmission and reception, and uses an actual aperture array by dividing the actual aperture array. The azimuth resolution improves in proportion to this number of divisions, and it is not necessary to make the traveling speed slow.
Here, an example of the vernier division method will be described with referring to FIGS. 9 and 10. Although the method in FIG. 9 is a usual synthetic aperture method, the method in FIG. 10 is a two-vernier division method where the actual aperture array is divided into two pieces. First, the usual synthetic aperture method will be described.
With referring to FIG. 9, in ping 1, a first transmission of a signal is first performed from an array 110 with a length D to the detection target 102 at an azimuth position S1, and receives a reflected (echo) signal from the detection target 102. Next, in ping 2, a second transmission of a signal is performed from the vernier 110 to the detection target 102 at a position (azimuth position S2) where the sonar travels D/2 from the ping 1, and receives a reflected signal from the detection target 102. Next, in ping 3, a third transmission of a signal is performed from the vernier 110 to the detection target 102 at a position (azimuth position S3) where the sonar travels D/2 from the ping 2, and receives a reflected signal from the detection target 102. This transmission and reception is repeated N times continuously, and synthetic aperture processing is performed on the basis of N-time input signals. In this manner, since transmission and reception is performed in one ping with the sonar traveling D/2, azimuth resolution RES becomes D/2 and a speed V of the array 110 becomes PRFxc2x7D/2. Moreover, since transmission and reception is performed with using a whole array, it is not possible to define a phase-equivalent overlap point, and hence it is difficult to correct fluctuation with the overlap method.
Next, the two-vernier division method where an actual aperture array is divided into two pieces will be described. With referring to FIG. 10, the array 110 in FIG. 9 is divided into two pieces in the two-vernier division method in FIG. 10, and hence, an array consists of a rear vernier 111 and a front vernier 112. Then, the rear vernier 111 (a sub array with a length of D/2) transmits a signal, and both the rear vernier 111 and the front vernier 112 receive a reflected signal from the detection target 102. At this time, an input signal of the rear vernier 111 is equal to a transmission and reception signal at an azimuth position S1,1 (a position where the sonar travels D/4 forward from a backmost portion of the rear vernier 111). In addition, an input signal of the front vernier 112 is equal to a transmission and reception signal at an azimuth position S1,2 (a position where the sonar travels D/4 forward from the azimuth position S1,1). This relation is called a Displaced Phase Center. Since the azimuth resolution RES becomes twice as many as a usual method, that is, D/4 by performing this transmission and reception whenever the sonar travels D/2, the speed V of the array 110 becomes PRFxc2x7D/2 similarly to the usual method. Moreover, it is not possible to define a phase-equivalent overlap point, and hence it is also difficult to correct fluctuation with the overlap method. In addition, an example of the method of dividing and using an actual aperture array is also disclosed in Japanese Patent No. 2803658.
On the other hand, a technical subject of a synthetic aperture sonar is what is weak in fluctuation of a sonar. FIG. 11 is an explanatory diagram of fluctuation of a sonar. This figure shows a case where a sonar 120 travels from the left-hand side in this figure to the right-hand side. The sonar 120 travels on a straight-line locus 121 ideally. However, the sonar 120, namely, a sonar platform such as a towed object, which is towed in underwater, or a sailing object that is self-propelled does not always travel on the ideal straight-line locus 121. Since fluctuating the influence of fluid turbulence or a tidal current, the sonar 120 travels on a curve as shown by a curve 121xe2x80x2 in fact.
Since time interval between transmission and reception is long in comparison with a radar in the synthetic aperture sonar having the acoustic velocity slow in underwater, a deviation amount 122 also becomes large. Consequently, the synthetic aperture array of SAS is distorted remarkably, and hence, an ideal beam synthesis cannot be performed for resolution to deteriorate. Since the brittleness against fluctuation is a very severe problem, letting the wave length of a center frequency be xcex, an image deteriorates even if the fluctuation is about 10% of xcex. Since this is equivalent to the fluctuation of about 1.5 cm even if the sonar is a low frequency sonar with a frequency of about 10 kHz, it is difficult to measure this at sufficient accuracy even if a present high-precision acceleration sensor is used.
In order to correct the fluctuation, various kinds of post processing called Auto-focusing is used in SAR. The basic principle of the Auto-focusing is a method of searching in inverse operation for a phase correction term with which a synthetic aperture image, whose resolution is deteriorated because of including the fluctuation (defocused), may be focused.
However, it is a precondition for this method that a xe2x80x9cfocused imagexe2x80x9d is known in foresight. For example, in SAR, an artificial structure such as a building and a street partition may serve as a guideline of focusing, and a method of arranging a strong radio wave scattering object in a mapping region beforehand may be used. In SAR mainly for ground information, such a method is applicable and effective.
On the other hand, generally, the presence of such an artificial structure is not expectable to the submarine surface information that SAS treats, and hence, a focused state cannot be predicted beforehand. In addition, a method of arranging an artificial acoustic wave scatterer beforehand is also conceivable. However, since an area of a map obtained by one sailing by a sonar with a slow acoustic velocity is very narrow, a sea bottom will become full of scatterers when a large area is mapped.
From such a background, approaches to the fluctuation correction, which is different from those of SAR, in SAS has been studied. A most typical method is a method of sequentially obtaining the fluctuation deviation amount between pings from an input signal in an overlap part by partially overlapping an actual aperture array every transmission and reception (R. S. Raven, xe2x80x9cElectronic Stabilization for Displaced phase Center Systemxe2x80x9d, U.S. Pat. No. 4,244,036 January 1981).
In consideration of the application to the above-described vernier division method, a method of traveling with making one of a plurality of space sampling points obtained by a vernier overlap is conceivable. Letting this method be a vernier overlap method, its outline is shown in FIG. 12.
With referring to FIG. 12, the structure of the array in the vernier overlap method is the same as that of the above-mentioned two-vernier division method (refer to FIG. 10), and hence, the same numbers as those are assigned and explanation of them will be omitted. In the vernier overlap method, a sonar travels D/4 every ping, and transmission and reception is performed.
Namely, first, a transmission and reception signal is obtained at azimuth positions S1, 1, and S1, 2 in ping 1. Next, a transmission and reception signal is obtained at azimuth positions S2, 1, and S2, 2 in ping 2 where the sonar travels D/4 from this ping 1. Therefore, the transmission and reception signal at the azimuth position S1, 2 in ping 1, and the transmission and reception signal at the azimuth position S2, 1 in ping 2 become transmission and reception signals in the same azimuth position. Hence, a deviation amount by a fluctuation amount is detectable by performing the cross correlation processing of these two transmission and reception signals, and hence, it becomes possible to correct fluctuation.
FIGS. 13A and 13B are explanatory diagrams of phase and fluctuation correction at an overlap point of pings 1 and 2. FIG. 13A shows reception time t1 of the signal 131 by the front array (vernier) 112 in ping 1, and FIG. 13B shows reception time t2 of the signal 132 by the rear array (vernier) 111 in ping 2. Since phase fluctuation is there between pings 1 and 2, reception time t1 and t2 does not coincide. Then, time delay and phase fluctuation are computed by taking cross correlation between the input signal 131 by the front array 112 and the input signal 132 by the rear array 111, and the fluctuation correction is performed.
Similarly, the transmission and reception signal at the azimuth position S2,2 in ping 2, and the transmission and reception signal at the azimuth position S3, 1 in ping 3 become transmission and reception signals in the same azimuth position. Hence, a deviation amount by a fluctuation amount is detectable by performing the cross correlation processing of these two transmission and reception signals, and hence, it becomes possible to correct fluctuation.
FIGS. 14A and 14B are explanatory diagrams of phase and fluctuation correction at an overlap point of pings 2 and 3. FIG. 14A shows reception time t3 of a signal 133 by the front array 112 in ping 2, and FIG. 14B shows reception time t4 of a signal 134 by the rear array 111 in ping 2. Since fluctuation is also there between pings 2 and 3, reception time t3 and t4 does not coincide. Then, time delay and phase fluctuation are computed by taking cross correlation between the input signal by the front array 112 and the input signal 134 by the rear array 111, and fluctuation correction is performed.
As apparent from the figure, the high resolution of D/4 and a fluctuation correction function are simultaneously realizable in the vernier overlap method. However, since space-sampling points are made to overlap, the sonar can travel only D/4 per ping. Namely, in the vernier overlap method, unless a range is shortening or the actual aperture length D is lengthened, only a half of the usual traveling speed will be obtained.
That is, a method in FIG. 12 is a method of applying the overlap fluctuation correction to the two-vernier division processing. Although azimuth resolution is twice as usual and fluctuation correction becomes possible, speed falls to a half of the usual speed since it is necessary to defining a phase-equivalent overlap point.
In addition, other examples of methods of each making an actual aperture array overlap to correct fluctuations are disclosed in Japanese Patent Laid-Open No. 3-218485, Japanese Patent Laid-Open No. 9-264959, Japanese Patent Laid-Open No. 11-337639, and Japanese Patent Laid-Open No. 11-344565.
A first problem is that a synthetic aperture sonar that performs fluctuation correction according to an overlap method is a slow traveling speed. That is because traveling distance per one-time transmission and reception becomes short in comparison with a usual case without overlapping since the sonar travels with making the actual aperture array overlap partially.
A second problem is a point that a range becomes short when a traveling speed is made to increase. That is because it is necessary to make R small in order to make V large as shown by the trade-off in formula (1). If V and R have an inverse proportion relation, area effectiveness will not be improved.
A third problem is a point that an actual aperture array is enlarged and hence spatial resolution falls, when a traveling speed is made to increase. That is because it is necessary to make D/2 large in order to make V large as shown by the trade-off in formula (1). Here, while being the length of an actual aperture array in the longitudinal direction, item (D/2) expresses azimuth spatial resolution. Enlargement of the actual aperture array decreases the employment property of a system remarkably, and deterioration of the spatial resolution means quality degradation of a map image obtained.
A fourth problem is a point the fluctuation correction processing by the overlap method becomes impossible when the traveling speed is increased. That is because the fluctuation deviation amount between pings is undetectable since an overlap part is not generated by the actual aperture array when traveling at the speed of equal sign conditions of formula (1).
Then, an object of the present invention is to provide a method of not reducing a range and spatial resolution, being able to perform fluctuation correction processing, and accelerating traveling speed. In addition, another object of the present invention is to provide a method of being able to perform multi-look processing without reducing resolution.
In order to solve these problems, the present invention is characterized in a synthetic aperture sonar which has an actual aperture array divided into two or more verniers, comprising transmitting and receiving means for transmitting and receiving an acoustic signal from each of the verniers whenever the synthesis aperture sonar travels a fixed distance, and cross correlation processing means which performs cross correlation processing of the acoustic signal that are transmitted from and received by two verniers and overlap before and after the traveling of the fixed distance.
According to the present invention, it becomes possible not to reduce a range and spatial resolution, to perform fluctuation correction processing, and to accelerate traveling speed.
Moreover, another aspect of the present invention is characterized in a synthetic aperture processing method which has an actual aperture array divided into two or more verniers, comprising a transmission and reception step of transmitting and receiving an acoustic signal from each of the verniers whenever the synthesis aperture sonar travels a fixed distance, and a cross correlation processing step of performing cross correlation processing of acoustic signals that are transmitted from and received by two verniers and overlap before and after the traveling of a fixed distance.
According to other aspects of the present invention, the same effectiveness as that of the present invention described above are demonstrated.